The impact of chlorine on chromium speciation in a laminar diffusion flame

Measurements of total chromium and hexavalent chromium have been obtained i n a laminar methane/air diffusion flame. Chlorine was added to methane in t he form of methyl chloride gas. A dilution-quenching probe sampled the emis sions from the flame. Particles were collected on a filter and vapor phase species were collected in impingers. All samples were analyzed for hexavale nt chromium with a spectrophotometric method. The filter sample was also an alyzed for total chromium by X-ray fluorescence. Total chromium added to th e flame was determined by gravimetric analysis of the chromium source, chro mium hexacarbonyl. Recovery of chromium of up to 64% was achieved. The addi tion of methyl chloride to the fuel had a dramatic impact on the speciation of the chromium emission. The fraction of hexavalent chromium increased si gnificantly as small amounts of chlorine were added. Partitioning between p hases was also altered; much more hexavalent chromium was found in the impi ngers as the chlorine loading increased. Equilibrium calculations reproduce d experimental trends but showed strong sensitivity to the thermochemical p roperties of the included species, particularly CrO2Cl2. (C) 2001 by The Co mbustion Institute.